Controllable Location of Inorganic Nanoparticles on Block Copolymer

Aug 22, 2017 - ... the head of one hemisphere, the entire hemisphere, or along the boundary between the two distinct hemispheres of the Janus-like sca...
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Controllable Location of Inorganic Nanoparticles on Block Copolymer Self-Assembled Scaffolds by Tailoring the Entropy and Enthalpy Contributions Nan Yan, Yan Zhang, Yun He, Yutian Zhu,* and Wei Jiang State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China S Supporting Information *

ABSTRACT: Precisely controlling the spatial location and alignment of functional nanoparticles (NPs) on polymeric scaffolds is of great importance to not only create novel nanostructures but also enhance the properties of the hybrid nanomaterials. Herein, we demonstrate a strategy of tailoring the entropic and enthalpic contributions to precisely position gold nanoparticles (AuNPs) on block copolymer (BCP) scaffolds through the confined coassembly of BCPs and AuNPs within the emulsion droplet. According to this strategy, entropic effect arisen by the loss in conformational entropy and the enthalpic attraction between ligands on AuNPs and surfactants at the oil/water interface induce the solid AuNPs to move to the BCP surface, while the enthalpic interaction between the ligands on AuNPs and the corresponding polymer chains guides the AuNPs to position at the appropriate place. By this strategy, both the location and alignment of AuNPs on BCP scaffolds can be controlled at will, such as at the two terminals or along the lamellar boundary of the pupa-like scaffolds, or at the bases of pinecone-like or bud-like scaffolds, or at the head of one hemisphere, the entire hemisphere, or along the boundary between the two distinct hemispheres of the Janus-like scaffolds. We believe that this methodology can offer a universal route to achieve the precise positioning of functional NPs on the BCP scaffolds.

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INTRODUCTION Doping various types of functional inorganic nanoparticles (NPs) into polymer matrix is a fascinating approach to engineer flexible hybrid nanocomposites with outstanding optical, electric, or magnetic properties, which have versatile applications in solar cells, sensors, target drug delivery, metal catalysis, and magnetic storage devices.1−9 In general, the integrated properties of the hybrid nanocomposites depend not only on the properties of the individual components but also on the spatial location and alignment of the functional NPs within the polymer matrix.10−17 For instance, Wang et al. reported that precisely controlling spherical gold nanoparticle array with sub10 nm spacing between adjacent nanoparticles made the nanocomposites exhibit high, stable, and reproducible surfaceenhanced Raman scattering activity.18 Quinten et al. achieved a linear chain of spherical metal nanoparticles in which light was transmitted by electrodynamic interparticle coupling. This linear chain structure of metal nanoparticles is very useful for subwavelength transmission lines with integrated optics circuits and for near-field optical microscopy.19 On the other hand, selfassembly of block copolymers (BCPs) confined in a small emulsion droplet can generate novel nanostructured particles with tunable internal structures, shapes, and surface properties, which can be the outstanding scaffolds to direct the spatial location of functional NPs as well as their alignment within the BCP scaffolds.20−30 Therefore, if we can achieve the precise © XXXX American Chemical Society

positioning of the introduced functional NPs on BCP scaffolds, we can not only design novel nanomaterials but also enhance the properties of the original hybrid nanomaterials.1,31−33 So far, however, it is still a great challenge to precisely control the position of the incorporated NPs on BCP scaffolds from the three-dimensional (3D) confined coassembly within the emulsion droplet, especially for the achievement of the alignment of NPs.20,21,25 For example, Ku et al. created the unique convex lens-shaped poly(styrene-b-4-vinylpyridine) (PS-b-P4VP) particles with highly ordered nanostructure and controlled gold nanoparticle (AuNPs) preferentially distributing at the surface of the P4VP domain because of the strong favorable interaction between the oleylamine-capped AuNPs and 3-n-pentadecylphenol.21 Jang et al. fabricated striped ellipsoidal particles from the self-assembly of poly(styrene-b2-vinylpyridine) (PS-b-P2VP) diblock copolymers confined in 3D droplet geometry and controlled AuNPs selectively distributing at the surface of P2VP stripes attributed to the favorable interaction between the AuNPs and P2VP chains.25 Although great efforts have been paid to this issue, there is rare successful example that can achieve the precise positioning of Received: May 23, 2017 Revised: July 18, 2017

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DOI: 10.1021/acs.macromol.7b01076 Macromolecules XXXX, XXX, XXX−XXX

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polymer matrix) and enthalpic effect (i.e., the interaction between AuNPs and each individual block, and the interaction between AuNPs and oil/water interface). As the organic solvent evaporates, the incorporated AuNPs inside BCP cause the conformational entropy loss of the polymer chains, which tends to repel the AuNPs to BCP scaffold/water interface. In addition, if there is interaction between the ligands on AuNPs and the surfactants, the attractive force will also help the AuNPs migrate to the BCP scaffold/water interface. On the other hand, selecting different ligands or mixing two different ligands to chemically modify AuNPs can tune the enthalpic interaction between AuNPs and each individual block, which can precisely control the positioning of AuNPs on the surface of BCP scaffold. Utilizing the entropy and enthalpy forces, a series of unique hybrid nanostructures with controllable distribution and alignment of AuNPs on various BCP scaffolds are tailored, such as controllable positioning AuNPs at the two terminals of the pupa-like scaffolds, or concentrating at the bases of pineconelike or bud-like scaffolds, or at the head of one hemisphere or covering the entire hemisphere of the Janus-like scaffolds, or parallel aligning along the interfacial boundary of the pupa-like and the Janus-like scaffolds, as illustrated in Scheme 1.

NPs on BCP scaffolds through the 3D confined coassembly of NPs and BCPs, such as the alignment or array of NPs. In general, both the enthalpic and entropic effects are involved when the inorganic NPs are introduced into the polymer matrix.2,34−37 Qualitatively, the overall change of the Gibbs free energy upon the incorporation of NPs into the polymer matrix can be described as follows: ΔG = (ΔHNP/polymer + ΔHsurface) − T (ΔScon + ΔStrans) (1)

where ΔHNP/polymer refers to the enthalpic energy between the incorporated NPs and polymer domains, which can be controlled by tailoring the properties of ligands on the surface of nanoparticle.38,39 ΔHsurface originates from the difference in the surface tensions when the surface components of the hybrid nanocomposites are changed. 2 Entropic energy mainly originates from the change in conformational entropy of the polymer chains (ΔScon) and the translational entropy of the introduced NPs (ΔStrans).2,35 When the solid NPs are immersed into polymer domain, the polymer chains must stretch around these obstacles, causing the loss in conformational entropy.40,41 Clearly, the entropic penalty increases with the nanoparticle radius.21 On the other hand, ΔStrans can be obtained by the uniform distribution of NPs within polymer domain.38,40 On the basis of eq 1, it is possible in theory to precisely position the additive NPs on the BCP scaffolds by tailoring the enthalpic and entropic contributions. Herein, we demonstrate a strategy of tailoring the entropy and enthalpy contributions to realize the precise positioning of NPs on various nanoscaled BCP scaffolds, which can precisely control not only the location of NPs, but also their alignments, as shown in Scheme 1. Specifically, the chloroform solution of

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RESULTS AND DISCUSSION In the current study, two types of symmetric diblock copolymers, i.e., PS-b-P2VP and PS-b-P4VP, were selected as the self-assembly units to coassemble with AuNPs within the emulsion droplet with a nearly neutral interface to generate hybrid nanoparticles. Typically, BCPs or the mixture of BCPs and AuNPs were dissolved in chloroform and then emulsified with an aqueous solution containing surfactants by ultrasonication or vigorous stirring. The chloroform in the emulsion droplet diffused through the water phase and slowly evaporated, leading to the self-assembly of BCPs or the coassembly of BCPs and AuNPs. After complete removal of chloroform, the glassy nanoparticles can be obtained. By selecting PS102k-b-P2VP97k diblock copolymer as the assembly unit, ellipsoidal pupa-like particles containing the alternated P2VP and PS lamellas were generated (see the Supporting Information, Figure S1). The surfactants used here are the mixture of cetyltrimethylammonium bromide (CTAB) and poly(vinyl alcohol) (PVA), which can well tailor the interfacial property of the emulsion droplets to a nearly neutral environment for both PS and P2VP blocks at a CTAB:PVA ratio of 1:3.41 To unveil the phase-separated structures of the PS102k-b-P2VP97k nanoparticles, the P2VP domains were selectively stained with iodine (I2) vapor, as shown in Figure S1b. From the transmission electron microscopy (TEM) images (Figure S1, parts a and b), it is calculated that the average thickness of PS and P2VP layers are ∼34.3 and 34.0 nm, respectively. The size distribution of the PS102k-b-P2VP97k nanoparticles was measured by dynamic light scattering (DLS), as shown in Figure S1c. Clearly, this ordered pupa-like particles can be the ideal scaffolds to load inorganic AuNPs via the confined coassembly of PS102k-b-P2VP97k and AuNPs. The synthesized PS2k-modified AuNPs (Au5.5S, 5.5 is the core diameter of the AuNPs; S represents the PS2k-SH ligands on the surface of AuNPs) were used to coassemble with PS102k-bP2VP97k via the same emulsion-solvent evaporation approach. The detailed information for the synthesis of Au5.5S can be found in the Supporting Information, section S2.1. After adding a small amount of Au5.5S-NPs (5.9 vol %, the volume fraction of Au5.5S to the sum of Au5.5S and PS blocks) to coassemble with

Scheme 1. Schematic Showing the Unique Hybrid Nanoparticles from the Co-Assembly of BCPs and AuNPs Confined in the Emulsion Droplets

AuNPs and BCPs is emulsified by ultrasonication or vigorous stirring in aqueous solution containing surfactants. Then, the cooperative self-assembly of BCPs and AuNPs confined in the 3D emulsion droplet is triggered when the organic solvent volatilizes and shrinks to a certain degree. During the confined coassembly of AuNPs and BCPs, the location and alignment of AuNPs on the nanoscaled BCP scaffolds can be well controlled by the entropic effect (i.e., entropic repulsion arisen by the loss in conformational entropy after incorporating AuNPs into B

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Figure 1. TEM images of hybrid Au5.5S/PS102k-b-P2VP97k nanoparticles incorporated with different volume fractions of Au5.5S fabricated by 3D confined coassembly within the emulsion droplets: (a) 5.9 vol %, pupa-like particles; (b) 20.0 vol %, pinecone-shape particles; (c) 33.3 vol %, budlike particles. The corresponding TEM images with P2VP stained by iodine (I2) vapor are inserted in the lower left corner.

Figure 2. TEM images of the Janus-like Au3.5S/PS195k-b-P4VP204k hybrid nanoparticles with (a−c) 26.92 vol % and (d−f) 55.11 vol % Au3.5S, respectively. Parts a and d and parts b and e are the TEM images of Au3.5S/PS195k-b-P4VP204k hybrid nanoparticles before and after staining the P4VP domain by I2 vapor, respectively. Parts c and f are the STEM images and the corresponding TEM images inserted in the lower left corner.

domain causes the loss of chain conformation of PS blocks. To reduce the loss of conformational entropy, PS blocks tend to expel the Au5.5S to the surface of PS domain. It has been proposed that D/L is a critical parameter to quantitatively characterize the entropic penalty, i.e., ΔScon, where D is diameter of the introduced NPs and L is the dimension of the compatible domain.20,40,43 Herein, the total diameter of Au5.5S (the sum of the Au core and PS shell) is 11.4 nm (Table S1, see the Supporting Information.) and the thickness of the PS lamellae in neat pupa-like PS102k-b-P2VP97k particles is ca. 34.3 nm. Therefore, the D/L value is 0.33. Since D/L is less than 0.5, it seems that only the entropic force cannot make the Au5.5S migrate from the interior to the surface of BCP scaffold. However, there is attractive interaction between PS ligands on the AuNPs and CTAB surfactant, which also promotes the migration of Au5.5S to the surface of BCP scaffold. Ultimately, these PS modified AuNPs aggregate together at the two terminals of the pupa-like particles, the bases of the pineconeshape or bud-like particles, to further reduce the loss of conformational entropy. In order to further confirm the above physical mechanism, we also synthesized the relatively small PS-coated AuNPs, i.e., Au1.7S (1.7 is the core diameter of the

PS102k-b-P2VP97k diblock copolymer confined in the emulsion droplet with a nearly neutral interface (CTAB:PVA = 1:3), it is observed that most of the Au5.5S-NPs are positioned at the two terminals of the pupa-like scaffolds, as shown in Figure 1a. When Au5.5S content is increased to 20.0 vol %, Au5.5S and PS102k-b-P2VP97k diblock copolymer coassemble into the particular pinecone-shape particles with the PS/P2VP alternated lamellae, while all the Au5.5S-NPs are selectively located at the base of the hybrid particles, as shown in Figure 1b. When the content of Au5.5S is further increased to 33.3 vol %, the hybrid nanoparticles transform into the bud-like structure, with all Au5.5S-NPs selectively distributing at the base of the buds (Figure 1c). This Au5.5S content controlled morphological transition from pupa-like particles to pinecone-like particles, and then to bud-like particles is mainly attributed to the attractive interaction between the PS ligands on the Au5.5S-NPs and CTAB, which makes the emulsion interface more selective to the PS domain.42 Moreover, it is observed that most of the Au5.5S NPs tend to aggregate together on the surface of BCP scaffolds rather than randomly distribute within the PS domain, which is attributed to the synergistic effect of entropy and enthalpy. First of all, the incorporation of solid Au5.5S into PS C

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Au3.5S NPs are aggregated at this area. When the volume fraction of Au3.5S is as high as 55.11 vol %, it is observed that the Au3.5S-NPs gradually cover the whole PS hemispheres, resulting in the formation of a large scale of the ordered hybrid organic/inorganic Janus-like nanoparticles, as shown in Figure 2d−f. In contrast to controllable distribution of NPs on a certain region of the BCP scaffold, the ability to position NPs in a linear alignment at the interface of the BCP scaffold is very limited. Up to now, there were only very limited reports that have successfully made an alignment of NPs in the BCP thin film.13,46−48 For example, Zhu and co-workers fabricated the hierarchically structured film with perpendicular orientation of the cylindrical domains and precisely controlled the AuNPs locating along the centerline of the cylinder phases.13 To the best of our knowledge, there is rare successful work in 3D confined coassembly that can precisely position NPs in a linear alignment at triphasic interface of the BCP scaffolds. In the following study, we synthesize the Au2.0SV nanoparticles coated with both the PS and P4VP ligands (2.0 is the diameter of Au core, S represents PS2k-SH ligands, while V represents P4VP2.5kSH ligands. Detailed information for the synthesis of Au2.0SV can be found in the Supporting Information, section S2.2.), which possess affinity with both PS and P4VP (or P2VP) blocks due to the enthalpic interactions. Subsequently, Au2.0SV and PS102k-b-P2VP97k are coassembled in emulsion droplets with a nearly neutral interface for both PS and P2VP blocks (CTAB:PVA = 1:3). From the TEM images of the resulting hybrid nanoparticles (Figure 3, parts a and b), it is clearly observed that nearly all of the Au2.0SV-NPs orderly align along the boundary of the PS and P2VP segments. Utilizing ethanol as the selective solvent (a good solvent for P2VP blocks but a

AuNPs, S represents the PS2k-SH ligands on the surface of AuNPs.) to coassemble with PS102k-b-P2VP97k block copolymer at the same experimental condition. Detailed information for the synthesis of Au1.7S can be found in the Supporting Information, section S2.1. The sum of the Au core and the PS shell is 6.0 nm (Table S1, see the Supporting Information) and the corresponding D/L value is 0.17, indicating that the entropic penalty can almost be ignored. It is clear that Au1.7S NPs are randomly distributed in the entire PS lamellae of the pupa-like hybrid Au1.7S/PS102k-b-P2VP97k particles, as shown in Figure S2. In addition, the well-defined Janus BCP particles with uniform nanostructures and narrow size distribution were also fabricated by the 3D confined self-assembly of PS195k-bP4VP204k diblock copolymer within the chloroform-in-water emulsion droplets (see the Supporting Information, Figure S3), which can be an excellent scaffold to load AuNPs to form Janus-like hybrid nanomaterials.44 The measurement of DLS shows that the PS195k-b-P4VP204k Janus particles possess a narrow size distribution and their average hydrodynamic diameter is ∼122 nm, as shown in Figure S3d. From the TEM images shown in Figure S3, it is calculated that the average length of PS domain along the axis direction is ∼49.75 nm. The formation of PS195k-b-P4VP204k Janus nanoparticles is attributed to the strong confinement effect and a nearly neutral emulsion/water interface for both PS and P4VP blocks. After introducing Au3.5S-NPs (3.5 is the diameter of the cores of AuNPs, S represents the PS2k-SH ligands) and PS195k-bP4VP204k diblock copolymers into the emulsion droplet, Janus-like hybrid nanomaterials were obtained, as shown in Figure 2. Detailed information for the synthesis of Au3.5S can be found in the Supporting Information, section S2.1. Parts a−c of Figure 2 show the TEM and scanning transmission electron microscopy (STEM) images of the Janus-like Au3.5S/PS195k-bP4VP204k hybrid nanoparticles containing 26.92 vol % Au3.5S. To further reveal the specific hierarchical nanostructure of the hybrid particles and distinguish the location of the Au3.5S on the PS195k-b-P4VP204k particles, the P4VP domains were selectively stained with I2 vapor, as shown in Figure 2b. It is clear that the PS modified Au3.5S nanoparticles are entirely selectively located on PS hemisphere. More interestingly, it is worth to note that these Au3.5S nanoparticles are not randomly dispersed in the PS hemispheres, but concentrated at the head of PS hemispheres, as pointed out by the red arrows in Figure 2a. Similar to the pupa-like hybrid particles shown in Figure 1a, there are several factors that contribute to the formation of this particular Janus-like Au3.5S/PS195k-b-P4VP204k hybrid particles (Figure 2a−c). First, enthalpic interaction between the PS blocks and PS ligands on the AuNPs makes Au3.5S have affinity with PS domain rather than P4VP domain. Second, the attraction between the PS ligands and CTAB surfactant drives the flexible Au3.5S NPs moving to the BCP surface during the evaporation of the organic phase.45 However, Au3.5S NPs are always confined in emulsion droplet and cannot diffuse into the aqueous solution because of the emulsion interfacial tension and the hydrophobicity of the AuNPs. Third, PS blocks are strongly compressed because of the strong confinement, which causes an entropic repulsion to the embedded Au3.5S NPs, thus also contributing to the migration of the flexible Au3.5S NPs to BCP surface. Finally, the Au3.5S NPs tend to aggregate together at the end of PS hemisphere after reaching the surface of PS hemisphere. This is because the end of the PS hemisphere is also the end of PS blocks, which causes less entropic penalty if

Figure 3. TEM images of hybrid Au2.0SV/PS102k-b-P2VP97k nanoparticles with Au2.0SV precisely located at the interfacial boundary between the PS and P2VP lamellae before (a) and after (b) slightly staining the P2VP domains by I2 vapor. Part c shows the TEM image of the hybrid nanodisks disassembled from hybrid Au2.0SV/PS102k-bP2VP97k nanoparticles by ethanol, while part d is the corresponding STEM image. D

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Figure 4. TEM images of the Au3.5SV/PS195k-b-P4VP204k hybrid Janus particles before (a) and after (b) staining the P4VP domains with I2 vapor. (c) Corresponding STEM image.

Figure 5. TEM images of the novel Au3.5SV/PS195k-b-P4VP204k hybrid nanoparticles with different number of bulges (n) fabricated by 3D confined coassembly within the emulsion droplets. (a) Janus hybrid particles, n = 1; (b) dumbbell-like hybrid particles, n = 2; (c) triangular-like hybrid particles, n = 3; (d) tetrahedron-like hybrid particles, n = 4.

In addition, we also synthesize Au3.5SV-NPs that are affinitive to both PS and P4VP to coassemble with PS195k-b-P4VP204k via the emulsion-solvent evaporation approach. Detailed experimental information for the synthesis of Au3.5SV-NPs can be found in the Supporting Information, section S2.2. From the TEM images in Figure 4, we can see that all the Au3.5SV-NPs present a linear alignment along the boundary between the two distinct hemispheres on the PS195k-b-P4VP204k Janus-like scaffolds. The TEM image with selectively staining the P4VP domain by I2 vapor (Figure 4b) and the corresponding STEM image (Figure 4c) further confirm that all the Au3.5SV-NPs precisely align along the equators of Janus colloidal particles. Similar to linear alignment of AuNPs on the pupa-like scaffolds (Figure 3), the linear alignment of AuNPs on Janus-like scaffolds is also attributed to the enthalpy and entropy contributions. On the other hand, it has been reported that the confinement degree plays an important role on the self-assembly of BCPs within the emulsion droplet, which can be utilized to create novel hybrid nanomaterials.26,49,50 In experiments, the confinement degree is highly relative to the size of emulsion droplet, which can be roughly tuned by the ultrasonic time and intensity. By tuning the confinement degree (i.e., the size of emulation droplet), some novel hybrid Au3.5SV/PS195k-bP4VP204k nanostructures with different number of bulges (n) are fabricated by the 3D confined coassembly within the emulsion droplets, as shown in Figure 5. Under strong confinement (i.e., the emulsion droplets are very small.), the typical hybrid Janus nanoparticles are fabricated (n = 1), as shown in Figure 5a. More interestingly, as the size of the emulsion droplets increases, the novel dumbbell-like (n = 2), triangular-like (n = 3), and tetrahedron-like (n = 4) hybrid nanostructures are observed, as shown in Figure 5, parts b−d,

nonsolvent for PS blocks) to partially disassemble the pupa-like particles results in a series of nanodisks, as shown in Figure 3, parts c and d. Interestingly, all Au2.0SV-NPs align along the upper or lower edges of the nanodisks to form two concentric necklace-like structures (The arrow pointed a nanodisk in Figure 3d, which can clearly see two well-defined concentric rings.), which further confirms that Au2.0SV-NPs are precisely positioned at the triphasic interface (the interface between PS, P2VP and aqueous solution), as illustrated by the inset in Figure 3b. It is known that the biphasic interface is a surface, while the triphasic interface is a line. Controllable distribution of NPs at the biphasic interface has been achieved in some previous works.2,22,45 For example, Jang et al. incorporated AuNPs into the pupa-like PS-b-P2VP scaffolds via the same emulsion-solvent evaporation approach and precisely controlled AuNPs selectively distributing at the surface of P2VP stripes, i.e., the interface between P2VP and aqueous solution.22 Jeon et al. fabricated the onion-like hybrid AuNPs/poly(styrene-bbutadiene) (PS-b-PB) particles and positioned the AuNPs at the interface between the PS and polybutadiene (PB) segments via the coassembly of PS-coated AuNPs and PS-b-PB diblock copolymer within the emulsion droplet.45 To the best of our knowledge, however, precisely positioning NPs at the triphasic interface to form a necklace-like structure has never been reported before. The achievement of controllable alignment of AuNPs on pupa-like PS-b-P2VP scaffolds is also highly relative to the enthalpy and entropy effects. The entropic penalty impels Au2.0SV-NPs to the surface of BCP scaffolds, while the enthalpic attractions between the PS/P4VP ligands on AuNPs and PS/P2VP domains direct Au2.0SV-NPs to position at the boundary between PS and P2VP stripes, resulting in the formation of necklace-like alignment of AuNPs, as illustrated in Figure 3b. E

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Figure 6. Schematic diagram to illustrate the entropy and enthalpy effects during the evaporation of the emulsion droplet. (a) AuNPs and BCPs are dispersed in the emulsion droplet. (b) AuNPs are migrated to the surface of BCP aggregate due to the entropic repulsion and the attraction between the PS ligands on AuNPs and CTAB; (c1 and c2) Microphase separation between PS and P4VP resulting in the formation of pupa-like scaffolds and the enthalpic attraction precisely positioning AuNPs on the right place.

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CONCLUSIONS In conclusion, we developed a strategy by a combination of entropic and enthalpic contributions to precisely control both the location and the alignment of NPs on the BCP selfassembled scaffolds. Here, entropic and enthalpic contributions are like the latitude and longitude coordinates on Earth. By tailoring these two “coordinates”, one can precisely control the position of NPs on the BCP scaffolds. With this strategy, NPs can be well controlled at two terminals or along the phase boundary of the pupa-like scaffolds, or at the bases of pineconelike or bud-like scaffolds, or at one hemisphere or along the boundary between the two hemispheres of the Janus-like scaffolds. Since this new methodology is based on the equation of Gibbs free energy, it can be a universal route for the design of well-defined hybrid nanomaterials with controllable nanostructures and enhanced properties.

respectively. Similar morphological evolution of diblock copolymers with the confinement degree has also been reported in the previous works.26,50 The formation of these patch-like nanostructures is attributed to the difference of the solubility parameters of PS and P4VP. Since the solubility parameter of chloroform (19.0 MPa1/2) is closer to the solubility parameter of PS (18.6 MPa1/2) than the solubility parameter of P4VP (22.5 MPa1/2), P4VP forms the cores of the micelles which eventually evolve into bulges on the particle surface during the evaporation of the chloroform (Figure 5).3,26,50 During the evaporation of chloroform, PS-blocks are more extended than P4VP-blocks in droplets, and PS-blocks shrink more remarkably after complete solvent removal, thus also facilitating the formation of P4VP-bulges. Finally, formation of P4VP-bulges on the outside can increase the hydrophilic areas of the particle surface to favor the decrease of interfacial free energy. In addition, it is worth to note that the Au3.5SV-NPs always present a linear alignment at the boundaries of the PS and P4VP domains even though the number of the bulges on hybrid nanoparticles is changed. To the best of our knowledge, these unique nanoscaled BCP/ AuNPs hybrid nanoparticles with tunable nanostructure and ordered alignment of AuNPs have never been reported before. To better understand the entropy and enthalpy effects, we select AuNPs/PS102k-b-P2VP97k pupa-like hybrid nanostructures as the example and draw a schematic diagram to illustrate how to precisely position AuNPs onto the BCP scaffolds via tailoring the entropy and enthalpy factors. As the organic solvent is approximately evaporated, the entropic repulsion arisen by the loss in conformational entropy of BCP chains and the enthalpic interaction between the PS ligands on AuNPs and CTAB at the oil/water interface cause AuNPs to migrate from the interior to BCP surface, as illustrated in Figure 6, parts a and b. As the organic solvent is completely evaporated, microphase separation between PS and P2VP blocks occurs, resulting to the formation of the pupa-like scaffolds. Accompanying with the microphase separation of BCPs, enthalpic attraction between the ligands on AuNPs and the corresponding polymer chains will guide AuNPs precisely to position at the right place on the pupa-like scaffolds, i.e., PS coated AuNPs are located at the two terminals of the pupa-like scaffolds (Figure 6c1) while PS/P4VP coated AuNPs are positioned at the triphasic interface of the PS, P2VP and aqueous phase (Figure 6c2). Moreover, there is relative less loss in conformational entropy for the PS chains if PS-coated AuNPs are located nearby the ends of PS chains (i.e., the two terminals of pupa-like scaffolds) instead of the surfaces of the other PS stripes, as illustrated in Figure 6c1.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01076. Experimental details including synthesis, sample preparation, and characterization, supplementary TEM images, and size analyses of the AuNPs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(Y.Z.) E-mail: [email protected]. ORCID

Yutian Zhu: 0000-0002-7092-0086 Wei Jiang: 0000-0001-6250-1327 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China for General Program (51373172), Major Program (51433009), and the Open Project of State Key Laboratory of Supramolecular Structure and Materials (sklssm201734).

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REFERENCES

(1) Balazs, A. C.; Emrick, T.; Russell, T. P. Nanoparticle Polymer Composites: Where Two Small Worlds Meet. Science 2006, 314, 1107−1110.

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DOI: 10.1021/acs.macromol.7b01076 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.7b01076 Macromolecules XXXX, XXX, XXX−XXX